Recent studies of the applicants support the role of the endogenous gaseous biological mediator hydrogen sulfide (H2S) in colorectal cancer: selective upregulation of cystathionine-b-synthase (CBS) and the subsequent production of H2S in colonic cancer cells serves as a pro-survival factor by stimulating tumor cell bioenergetics, growth, proliferation, migration and invasion. This work identifies CBS as a novel antitumor target. In order to advance the clinical translation of this concept, we will pursue the following two Aims: Aim #1. To conduct screening a composite library of clinical drugs, drug-like compounds and pharmacologically active molecules (>5,000 compounds) to identify novel pharmacological inhibitors of CBS, with the aim of drug repurposing for cancer therapy. In Aim #2, Inhibitors identified in the cell-based screen will subsequently enter specificity and selectivty screening, followed by bioenergetics studies and studies of tumor cell proliferation in vitro. Successful completion of the current project will be evidenced by the identification of at least one clinically used drug (already used for non-oncological indications), which exerts CBS inhibitory and anti-proliferative effects, at concentrations that are therapeutically achievable in patients. Successful identification of such compound will trigger a Phase II STTR project, aimed at in vivo PK/PD studies and preclinical IND-enabling studies, culminating in the clinical repurposing of the lead compound. The applicant team (the PI at CBS Therapeutics and the head of the subcontract at the University of Texas Galveston) has all necessary theoretical and practical expertise to conduct the proposed work.